1. Field of the Invention
The invention relates to a bias circuit, and in particular to a bias circuit for biasing a hands-free module comprising a microphone and a headphone.
2. Description of the Related Art
FIG. 1 shows a conventional microphone bias circuit 10. An amplifier 12 is coupled to and drives a transistor 14, which is an NMOS transistor. The output voltage VOUT is generated at the source of the transistor 12, and the maximum the output voltage VOUT can reach is approximately (VDD−VGS−VDS).
For a microphone bias circuit, low output impedance may be desired, which typically involves using a source-follower output stage, e.g. class AB amplifier. FIG. 2 shows a conventional source-follower amplifier circuit 20. Rather than the NMOS transistor shown in FIG. 1, a PMOS transistor is used for the transistor 24 so the output voltage Vout increases close to the positive voltage VDD. However, the efficiency of the half source-follower amplifier circuit 20 is poor because current I1 must be the maximum amount regardless of current ILOAD draw from the load, which in telecom applications typically varies from zero to 1.3 mA to support up to the external electret microphones, for example. The source-follower amplifier circuit 20 requires a certain amount of current I1 from the power supply VDD and also requires a wide voltage range to work properly.
In a source-follower amplifier circuit such as the one shown in FIG. 2, the output VOUT tracks the input at VIN. If the input VIN goes high, the output VOUT also goes high. FIG. 2 is a class A type design which requires the current I1 to be set to the maximum load current ILOAD plus the current necessary to bias transistor 24 correctly. Even if ILOAD is removed, current I1 is still set to its maximum level, thus using 1-2 mA of current unnecessarily. Also, using a conventional 2-stage common-source amplifier 22 with a PMOS transistor 24 as the output device to obtain high output voltage and good efficiency creates a relatively high output impedance and poor PSRR (Power Supply Rejection Ratio). A large amount of semiconductor space is required for the compensation capacitor (not shown) coupled to the output. Another disadvantage of the conventional source-follower amplifier circuit 20 is that the output impedance is approximately equal to the output transconductance Gm of the transistor 24. A higher current I1 is required to accommodate the higher transconductance Gm.
FIG. 3 shows another conventional microphone bias amplifier circuit 30 as disclosed in U.S. Pat. No. 6,275,112. The output stage 32 of conventional microphone bias amplifier circuit 30 includes source-follower transistor 36 coupled to feedback loop 34 that keeps the source-follower transistor 36 current constant. The microphone of a cellular phone may be integrated in a hands-free module. A mono hands-free module comprises an earphone terminal, a microphone terminal, and a ground terminal, while a stereo hands-free module comprises right and left audio channel terminals, an earphone terminal, a microphone terminal, and a ground terminal. Note the earphone and microphone modules share the common ground terminal as their grounding.
The output of an audio buffer, which drives audio channels, is coupled to the headphone by alternating current (AC) coupling to block direct current (DC). However, some audio signals with low frequency are also suppressed. To solve this problem, a capacitor with large capacitance can be employed in the AC coupling stage to lower the high pass cut-off frequency below about 20 Hz. However, the large capacitor increases the size and cost of the circuit.
Another solution is to drive the headphone using DC coupling, wherein a reference voltage VCM exceeding the ground voltage is provided to the common ground terminal as the grounding for the headphone. However, since the earphone and microphone modules share the common ground terminal as their grounding, the voltage difference across the microphone module is decreased while replacing the ground voltage by the reference voltage VCM. As the system voltage becomes decreased, the voltage difference across the microphone module may not support the operation of the microphone module.